One of the Daves you knowwrote a review of POC Fornix Helmet on January 2, 2017
Familiarity: I've used it once or twice and have initial impressions
My head measures about 57.5cm +/-, depending on where the measuring tape sits, and the M/L fits perfectly. I was a bit worried about the sizing because POC apparently fits a round head better than an oval one and I'm guessing I'm more in the oval camp. I had the opportunity to briefly try on both the Receptor Bug and Fornix Backcountry in the same size before ordering and my memory is that the Receptor felt roomier than the Fornix whereas the Fornix Backcountry felt snug, particularly around the ear pieces. Not sure if there's an actual difference in internal shape/roundness between the three helmets or if it's just differences in padding but that was my experience.
I wear Anon M2 goggles and they pair well. No gap.